A foamed product made of a thermoplastic resin can be imparted with characteristics such as a lightweight property, a shock-absorbing property, a thermal insulating property, and the like, and is widely used as a packaging material, a building material, a thermal insulation material, and the like.
Above all, a foamed product made of a styrene resin is suitable as a packaging material, a building material, and a thermal insulation material because it is economical and possesses good expandability, and moreover is excellent in thermal insulating property, heat-retaining property, and stiffness. However, the foamed product made of a styrene resin has a drawback that it is inferior in heat resistance and a restoring property. Therefore, in applications such that the heat resistance and the restoring property are required, there is being used a foamed product made of, for example, a propylene resin.
On the other hand, the foamed product made of a propylene resin is inferior in stiffness compared to the foamed product made of a styrene resin. Furthermore, in order to secondarily expand as well as to mutually fuse together the propylene resin expanded beads in in-mold molding, there is needed heating at a higher temperature, namely heating with steam having higher saturated vapor pressure compared to in-mold molding of the polystyrene resin expanded beads. Therefore, a mold of high-pressure specification and a dedicated molding machine with high press pressure become necessary and, as a result, the cost of energy to be used also increases.
In order to solve these problems, there is proposed a molded article of expanded composite resin beads including a propylene resin component and a styrene resin component.
For example, in Patent Document 1, there is proposed a styrene-modified propylene resin obtained by impregnating a polypropylene resin with vinyl monomers followed by graft polymerization, wherein the ratio of [the content of the polypropylene resin]/[the content of a polymer of the vinyl monomers] is 95-60% by weight/5-40% by weight.
Furthermore, in Patent Documents 2 and 3, there is proposed a styrene-modified propylene resin obtained by impregnating a polypropylene resin with vinyl monomers followed by graft polymerization, wherein the ratio of [the content of the polypropylene resin]/[the content of a polymer of the vinyl monomers] is 50-20% by weight/50-80% by weight.
Further, in Patent Document 4, there is proposed a styrene-modified propylene resin wherein the ratio of [the content of the polypropylene resin]/[the content of the polymer of the vinyl monomers] is 76.9-14.3% by weight/23.1-85.7% by weight.